As energy is applied to a material continuously, the material will experience a temperature rise and then change from a solid state to a liquid state and finally to a gaseous state. Continuous application of energy will cause further changes of the state of the material. Specifically, electrons with negative charges, positive or negative ions, or other substances may be generated by neutral atoms or molecules of the gas due to high-energy collisions. The mixture of these substances with charges is collectively termed as “plasma”.
As a new kind of surface treatment technology, atmospheric plasma treatment such as plasma cleaning may accomplish a treatment at a low temperature and a normal atmospheric pressure without causing damage to the surface that is treated. Furthermore, the atmospheric plasma treatment requires no use of electric arcs, a vacuum chamber and a harmful gas exhausting system, so it has a low cost and presents no harm to the operators' health even after an extended time of operation. Therefore, the atmospheric plasma treatment has found wide application in a wide variety of industries, for example, in surface treatment of TFT-LCD substrates.
The atmospheric plasma discharging belongs to a dielectric barrier discharging (DBD) mode, i.e., an unbalanced-state gas discharging mode in which an insulation medium is inserted into a discharging space, and is also called a dielectric barrier corona discharging or a silent discharging. The DBD is able to operate at a high atmospheric pressure and a very wide frequency range, and usually occurs at an atmospheric pressure of 104˜106 Pa and a power frequency of 50 Hz to 1 MHz. Electrodes used for atmospheric plasma discharging may be designed in various forms. A certain working gas is filled between two discharging electrodes, and an insulation medium is applied to one or both of the electrodes or is suspended directly in the discharging space. Alternatively, a particulate medium is filled in the discharging space. Then, when an alternating current (AC) voltage that is sufficiently high is applied between the two electrodes, the gas between the electrodes will be broken down to cause discharge, i.e., the dielectric barrier discharging occurs. In practical applications, a pipeline-type electrode structure is widely used in various chemical reactors, while a planar electrode structure is widely used in modification, grafting, surface tension improvement, cleaning and hydrophilic modification of polymeric or metallic films and plates.
A conventional atmospheric plasma apparatus primarily includes an anode, an insulation medium and a cathode. In order to remove the plasma produced between the anode and the cathode, a plurality of plasma removing regions are typically formed in the cathode. Referring to FIG. 1, there is shown a schematic structural view of a cathode of the conventional atmospheric plasma apparatus. The cathode includes a plurality of plasma removing regions 401 and a plurality of plasma generating regions 403. For the atmospheric plasma apparatuses currently available in the market, the plasma removing regions of the cathode are mostly designed in a circular form and in a staggered arrangement.
However, the circular-form design and the staggered arrangement give rise to the following problems:
(1) the circular-form design fails to take into consideration the fact that the plasma generating regions and the plasma removing regions shall be equal to each other in area. Consequently, the plasma tends to be retained in the space between the anode and the cathode to cause erosion to the cathode and the insulation medium, thus shortening the service life of the apparatus; and
(2) in the staggered arrangement, the plasma removing regions and the plasma generating regions are unequally spaced. As a result, plasma near the plasma removing regions is easy to be removed while that away from the plasma removing regions is not. Thus, the plasma tends to be retained in local regions, and furthermore, a plasma concentration near the plasma removing regions outside the cathode becomes higher than a plasma concentration away from the plasma removing regions. This leads to a non-uniform distribution of the plasma, thus resulting in poor uniformity of the surface treatment.